Integrated Geophysical Investigation for Engineering Site Characterizations of Dam Construction Site, Near Wukro Town, Tigray, Northern Ethiopia
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Date
2012-06
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Addis Ababa Universty
Abstract
Integrated geophysical investigations using 2D electrical imaging, vertical electrical sounding and
magnetic methods have been conducted for engineering site characterization of a dam foundation
located at Hizaeti Afras near Wukro town, Tigray Regional State, North Ethiopia. The main
objective of the study was to examine the suitability of the subsurface geological formations and
structures for foundation of the proposed dam project.
From the results of the survey, it has been possible to map the stratification of the subsurface
layers, their vertical and lateral extent, and hence the depth to the sound bedrock suitable for dam
foundation is determined. It has also been possible to map areas of weakness in the subsurface that
could be detrimental to the life of the dam.
According to the interpretation of the geophysical results, the near surface geology of the study
area includes three layers. The first layer which is the top soil has considerably varying thickness
(2m-9m) over the area. The second layer is interpreted as a weathered and fractured limestone
with shale intercalation. And it extends to depth of about 27m on the western and 33m on the
northern part of the survey area. However, its depth extent on the eastern and southern part is
about 17m to 21m. The third layer which is characterized by higher apparent resistivity response is
interpreted as black limestone which could be considered as the bed rock in the area. This layer
has undulating nature and the depth to this bed rock surface is deeper in the western and northern
portions and become shallow towards the eastern and southern portions of the survey area.
Based on the pseudo-depth and sliced-stacked depth maps the study area can be divided in to three
distinct zones. The first zone which is characterized by very low apparent resistivity values
includes the western and northeastern portions of the study area. This is interpreted to be the
response of the water saturated weathered and fractured limestone. The second zone that exhibits
intermediate apparent resistivity values comprises of the central part of the study area with a
narrow strip in the central-east. This area is interpreted to be covered by weathered and fractured
limestone with shale intercalation. The third zone which is characterized by very high apparent
resistivity response comprises of the southeastern part of the area and it is interpreted as a black
limestone formation.
From a combined interpretation of the geophysical data, four weak zones have been identified
over the survey area which are oriented in N-S, NE-SW, NW-SE and E-W direction. The N-S
running major weak zone is in the region (about 100m -110m from the western side of the dam
axis) where an earlier dam built on the site has broken through.
As a further conclusion from the work, it is suggested that the dam axis has to be relocated
towards the downstream side by about 150m from the intended location. This relocated dam axis
will get the sound foundation at shallower depth and has a reduced risk from the weak zones
trending NE-SW and NW-SE. In addition special attention should be given to the western and
northeastern part of the study area and the weak zones in designing and implementation of the dam
foundation.
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Keywords
Wukro Town, Tigray